Temperature compensated pressure sensor

ABSTRACT

A pressure sensor ( 30 ) for environments with wide ranging temperature variations such as vehicle tires. The sensor has a sensor membrane ( 53 ) exposed to the fluid pressure to be sensed, and a reference membrane subject to a predetermined pressure difference. Both membranes are conductive and connected to circuitry that converts the deflection in the sensor membrane into an output signal indicative of the pressure difference across the sensor membrane, and converts the deflection of the reference membrane into a temperature compensation value used to adjust the output signal into a temperature compensated signal indicate of the fluid pressure to be sensed.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 10/965,922 filed on Oct. 18, 2004, now U.S. Pat. No. 7,093,494 all of which is herein incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to a pressure sensor and in particular, a micro-electro mechanical (MEMS) pressure sensor.

CO-PENDING APPLICATIONS

Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention simultaneously with the present application:

10/965,901 10/965,902 10/965,903 10/965,904 10/965,927 10/965,718 10/965,746 10/965,747 10/965,717 10/965,928 10/965,899 The disclosures of these co-pending applications are incorporated herein by cross-reference.

CROSS REFERENCES TO RELATED APPLICATIONS

The following patents or patent applications filed by the applicant or assignee of the present invention are hereby incorporated by cross-reference.

10/868,866   6,716,666  6,949,217  6,750,083 10/129,504  6,991,207  6,777,259  6,557,978  6,923,524  6,766,998 10/853,270  6,759,723  6,967,354  6,870,259  6,925,875 10/898,214  11/242,916

BACKGROUND ART

The invention has wide-ranging application across many fields of industry. It is particularly suited to pressure measurement in harsh or dynamic environments that would preclude many other pressure sensors. These applications include, but are not limited to:

-   -   monitoring engine pressure (cars, aircraft, ships, fuel cells)     -   sensors for high speed wind tunnels     -   sensors to monitor explosions     -   sensors for boilers     -   sensors for dish-washing machines     -   sensors for irons (both domestic and industrial)     -   sensors for other steam based machines where overpressure can         lead to destruction and loss of life         However, in the interests of brevity, the invention will be         described with particular reference to a tire pressure monitor         and an associated method of production. It will be appreciated         that the Tire Pressure Monitoring System (TPMS) described herein         is purely illustrative and the invention has much broader         application.         Transportation Recall Enhancement, Accountability and         Documentation (TREAD) legislation in the United States seeks to         require all U.S. motor vehicles to be fitted with a tire         pressure monitoring system (TPMS). This is outlined in U.S.         Dept. of Transportation, “Federal Motor Vehicle Safety         Standards: Tire Pressure Monitoring Systems; Controls and         Displays”, US Federal Register, Vol. 66, No. 144, 2001, pp.         38982–39004. The impetus for this development comes from recent         Firestone/Ford Explorer incidents which led to a number of fatal         accidents. A careful assessment of tire inflation data found         that approximately 35% of in-use tires are under inflated,         whilst an assessment of the effect of a TPMS found that between         50 to 80 fatalities, and 6000 to 10,000 non-fatal injuries, per         annum could possibly be prevented. This is discussed in U.S.         Dept. of Transportation, “Tire Pressure Monitoring System,”         FMVSS No. 138,2001. European legislation also appears likely to         require the fitting of a TPMS to increase tire life, in an         effort to reduce the number of tires in use by 60% in the next         20 years, so as to minimise the environmental impacts.         Two different kinds of TPMS are currently known to be available         in the marketplace. One kind of TPMS is based on differences in         rotational speed of wheels when a tire is low in pressure. The         asynchronicity in rotational speed can be detected using a         vehicle's anti-braking system (ABS), if present. The second kind         of TPMS measures tire pressure directly and transmits a signal         to a central processor. FIG. 1 (prior art) illustrates a         schematic of a typical pressure measurement based TPMS 10.         Sensors 12, provided with a transmitter, measure pressure in         tires 13 and transmit a signal 14 to antenna 16. The data can         then be relayed to a receiver 15 and processed and displayed to         a driver of the vehicle 17 on display 18.         Table 1 lists some presently known TPMS manufacturers/providers.         Motorola and Pacific Industries have each developed a TPMS,         whilst other companies listed in Table 1 act as suppliers for         TPMS manufacturers, including some automobile producers that         install their own TPMS.

TABLE 1 Pressure sensor manufacturers involved in TPMS. Company Supplier to Type of Sensor Motorola Motorola Capacitance Pacific Industries Pacific Industries Piezoresistive SensoNor Siemens, TRW, Bern, Piezoresistive Porsche, BMW, Ferrari, Mercedes, Toyota Siemens Goodyear Piezoresistive Transense Technologies Under development Surface Acoustic Wave TRW/Novasensor Smartire, Michelin, Piezoresistive Schrader, Cycloid There are two main types of pressure sensor; resistive or capacitive. Both types of these sensors rely on deflection of a membrane under an applied pressure difference. One side of the membrane is exposed to internal pressure of a tire while the other side of the membrane forms one wall of a sealed cavity filled with gas at a reference pressure. The resistive-type sensors typically employ silicon-based micro-machining to form a Wheatstone bridge with four piezoresistors on one face of the membrane. The sensor responds to stress induced in the membrane. For capacitive-type sensors, the membrane forms one plate of a capacitor. In this case, the sensor responds to deflection induced in the membrane. Preferably, the responses should be linear with pressure, for predicability, up to at least a critical point. Transense Technologies, listed in Table 1, have developed a different type of sensor, based on surface acoustic wave detection. This sensor relies on interferometric measurement of the stress-induced deflection of a reflective membrane. A fibre-optic cable both transmits and receives laser light, with one end of the fibre-optic cable being inserted into the interferometer. This system is discussed in Tran, T. A. Miller III, W. V., Murphy, K. A., Vengsarkar, A. M. and Claus, R. O., “Stablized Extrinsic Fiber Optic Fabry-Perot Sensor for Surface Acoustic Wave Detection”, Proc. Fiber Optic and Laser Sensors IX, SPIE vol. 1584, pp 178–186, 1991. Presently, there are also a variety of different kinds of deployment means for sensors in a TPMS, including valve cap and valve stem based systems, systems with the sensor mounted on the wheel rim or wheel hub, and also a tire-wheel system developed by an alliance of several tire manufacturers which has a sensor embedded in the wheel frame itself. These different kinds of deployment in TPMS are listed in Table 2.

TABLE 2 Specifications of TPMS in production. AM = products fitted to a vehicle after vehicle purchase (After Market). Warning Company/ Type of Level Accuracy Group System Fitted to (psi) (psi) Sampling Beru Wheel Rim Audi, BMW, user set 1 every 3 sec, Mercedes transmitted every 54 sec Cycloid Wheel Cap Ford, 18 1 30 sec/10 (pump) Goodyear min Fleet Valve Cap heavy 20 1 3.5 sec vehicles Johnson Valve Stem AM 19.9 1 15 min Michelin/ PAX Renault, ? ? ? Goodyear/Pirelli/ System Caddillac Dunlop Motorola Wheel Rim AM ? ? 6 sec Omron Valve Stem AM ? ? ? Pacific Industries Valve Stem AM 20.3/user 1.8 15 sec/10 set min Schrader Valve Stem Corvette, 22 2% ? Peugeot, Cadillac Smartire Wheel Rim Aston ? 1.5 6 sec Martin, Lincoln, AM To increase battery life, most TPMS are in stand-by mode for the majority of time, only operating at set intervals. The U.S. legislation requires the system to alert the driver within a set time of detecting significant tire under-inflation conditions. It also requires a warning light to signal when the tire is either 20% or 25% under-inflated. Most of the devices presently available in the market are accurate to within ±1 psi, which represents ±3% for a tire pressure of 30 psi. More generally, the sensor should perform in a harsh environment, with temperatures up to 130° C. and accelerations of 1000 g or more. Tire pressure increases and decreases in response to corresponding changes in temperature. Most systems presently available include a sensor to account for thermally induced changes in tire pressure sensor sensitivity (Menini, Ph., Blasquez, G., Pons, P., Douziech, C. Favaro, P. and Dondon, Ph., “Optimization of a BiCMOS Integratetd Transducer for Self-Compensated Capacitive Pressure Sensor,” Proc. 6^(th) IEEE Int. Conf. Electronics, Circuits and Systems, Vol 2, pp. 1059–1063, 1999). As TPMS's become standard on all motor vehicles, there is a clear imperative to produce low cost, reliable, robust sensors that meet or exceed the statutory requirements. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the chamber         containing a fluid at a reference pressure, such that the         flexible membrane deflects from any pressure difference between         the reference pressure and the fluid pressure, the membrane         being at least partially formed from conductive material; and,     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; wherein,     -   the chamber, the flexible membrane and the associated circuitry         are formed on and through a wafer substrate using         lithographically masked etching and deposition techniques.

The lithographically masked etching and deposition techniques used in the semiconductor chip manufacturing industry can produce many separate devices from a single wafer with high yields and low defect rates. Applying these fabrication techniques to MEMS pressure sensors and associated CMOS circuitry allows high volumes and high yields that dramatically reduce the unit cost of individual sensors.

A first related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the chamber         containing a fluid at a reference pressure, such that the         flexible membrane deflects due to pressure differentials between         the reference pressure and the fluid pressure; and,     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; wherein,     -   the membrane is less than 0.1 grams.

Designing and fabricating the sensor to minimize the mass of the membrane decreases the effects of acceleration on the membrane deflection. At the same time, a low mass has no effect on the membrane deflection from the pressure differential between the reference fluid and the air pressure.

A second related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   first and second chambers having first and second flexible         membranes respectively, the first and second flexible membranes         configured to deflect in response to pressure differences within         the first and second chambers respectively, the first membrane         arranged for exposure to the fluid pressure and the second         membrane sealed from the fluid pressure; and,         associated circuitry for converting the deflection of the first         flexible membrane into an output signal related to the fluid         pressure, and converting the deflection of the second membrane         into an adjustment of the output signal to compensate for the         temperature of the sensor.

By sealing the second chamber from the tire pressure, the deflection of the second membrane can be determined as a function of temperature. This can be used to calibrate the output signal from the first chamber to remove the effects of temperature variation.

A third related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the chamber         containing a fluid at a reference pressure, such that the         flexible membrane deflects due to pressure differentials between         the reference pressure and the fluid pressure; and,     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; wherein,         the membrane is at least partially formed from a conductive         ceramic material.

Conductive ceramics, such as metal ceramics, have previously been used to coat tool steels because of its corrosion and wear resistance. Surprisingly, it can be deposited as a thin membrane with sufficient flexibility for sensing pressure while retaining its corrosion and wear resistance. Furthermore, these materials are generally well suited to micro fabrication processes and electrically conductive so they can be used in capacitative and resistive type pressure sensors.

A fourth related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a first wafer substrate with a front side and an opposing back         side, a chamber partially defined by a flexible membrane formed         on the front side and at least one hole etched from the back         side to the chamber;     -   a second wafer on the back side of the first wafer to seal the         at least one hole; wherein,     -   the chamber contains a fluid at a reference pressure, such that         the flexible membrane deflects due to pressure differentials         between the reference pressure and the fluid pressure; and,     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; wherein,         the second wafer is wafer bonded to the first wafer substrate.

Wafer bonding offers an effective non-adhesive solution. It provides a hermetic seal with only minor changes to the fabrication procedure. Skilled workers in this field will readily understand that the most prevalent forms of wafer bonding are:

-   -   direct wafer, or silicon fusion, bonding;     -   anodic, or electrostatic Mallory process bonding; and,     -   intermediate layer bonding.

These forms of wafer bonding are discussed in detail below, and they all avoid the unacceptable air permeability associated with adhesive and polymer coatings.

A fifth related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the chamber         containing a fluid at a reference pressure, such that the         flexible membrane deflects due to pressure differentials between         the reference pressure and the fluid pressure; and,     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; wherein,     -   the membrane is non-planar.

It is possible to extend the linear range of the pressure-deflection response with a non-planar membrane. Corrugations, a series of raised annuli or other surface features are an added complexity in the fabrication process but can extend the linear range of the sensor by 1 MPa.

A sixth related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the flexible         membrane at least partially formed from conductive material, and         the chamber containing a fluid at a reference pressure, such         that the flexible membrane deflects from any pressure difference         between the reference pressure and the fluid pressure;     -   a conductive layer within the chamber spaced from the flexible         membrane; and, associated circuitry incorporating the flexible         membrane and the conductive layer; such that,     -   the conductive layer and the flexible membrane form capacitor         electrodes and the deflection of the flexible membrane changes         the capacitance which the associated circuitry converts into an         output signal indicative of the fluid pressure; wherein,     -   the conductive layer is arranged such that deflection of the         membrane towards the conductive layer can displace the fluid         from between the membrane and the conductive layer.

Venting the fluid between the electrodes to the other side of the fixed electrode, while keeping the chamber sealed, avoids the extreme fluid pressure that cause the squeeze film damping.

A seventh related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the flexible         membrane is a laminate having at least two layers wherein at         least one of the layers is at least partially formed from         conductive material, and the chamber containing a fluid at a         reference pressure, such that the flexible membrane deflects         from any pressure difference between the reference pressure and         the fluid pressure; and,     -   associated circuitry for converting deflection of the flexible         membrane into an output signal indicative of the fluid pressure.

Forming the membrane from a number of separately deposited layers alleviates internal stress in the membrane. The layers can be different materials specifically selected to withstand harsh environments.

An eighth related aspect provides a method of fabricating pressure sensor for sensing a fluid pressure, the method of fabrication comprising:

-   -   etching a recess in a wafer substrate;     -   depositing a flexible membrane to cover the recess and define a         chamber such that during use the chamber contains a fluid at a         reference pressure and the flexible membrane deflects from a         pressure difference between the reference pressure and the fluid         pressure;     -   depositing associated circuitry for converting the deflection of         the flexible membrane into an output signal indicative of the         fluid pressure; and,     -   depositing an apertured guard over the membrane.

An aspect closely related to the eighth aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a wafer substrate with a recess;     -   a flexible membrane covering the recess to define a chamber         containing a fluid at a reference pressure, such that the         flexible membrane deflects due to pressure differentials between         the reference pressure and the fluid pressure;     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; and,         an apertured guard over the membrane formed using         lithographically masked etching and deposition techniques.

By depositing material over the membrane to form the guard offers greater time efficiency and accuracy than producing a guard separately and securing it over the membrane. Semiconductor etching and deposition techniques allow highly intricate surface details. The apertures in the guard can be made smaller to exclude more particles from contacting the membrane. The fine tolerances of lithographic deposition permit the guard to be positioned close to the membrane for a more compact overall design.

A ninth related aspect provides a pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the chamber         containing a fluid at a reference pressure, such that the         flexible membrane deflects due to pressure differentials between         the reference pressure and the fluid pressure, the membrane         being at least partially formed from conductive material;     -   a conductive layer within the chamber spaced from the flexible         membrane such that they form opposing electrodes of a capacitor;         and,     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; wherein,     -   the conductive layer is less than 50 microns from the membrane         in its undeflected state.

A capacitative pressure sensor with closely spaced electrodes can have small surface area electrodes while maintaining enough capacitance for the required operating range. However, small electrodes reduce the power consumption of the sensor which in turn reduces the battery size needed for the operational life of the sensor.

A tenth related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the chamber         containing a fluid at a reference pressure, such that the         flexible membrane deflects from any pressure difference between         the reference pressure and the fluid pressure, the membrane         being at least partially formed from conductive material; and,     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; wherein,     -   the flexible membrane is less than 3 microns thick.

The operational range of the pressure sensor requires the membrane to have a certain deflection. For a given material, the deflection of the membrane will depend on, inter alia, its area and its thickness. Minimizing the thickness of the membrane allows the use of a high yield strength membrane material. A thinner membrane also allows the area of the membrane to be reduced. Reducing the area of the membrane reduces the power consumption and the overall size of the sensor. A high yield strength material is better able to withstand the extreme conditions within the tire and a compact design can be installed in restricted spaces such as the valve stem.

An eleventh related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:

-   -   a chamber partially defined by a flexible membrane, the chamber         containing a fluid at a reference pressure, such that the         flexible membrane deflects from any pressure difference between         the reference pressure and the fluid pressure, the membrane         being at least partially formed from conductive material; and,     -   associated circuitry for converting the deflection of the         flexible membrane into an output signal indicative of the fluid         pressure; wherein,         the associated circuitry is adapted to be powered by         electromagnetic radiation transmitted from a point remote from         the sensor.

Beaming energy to the sensor removes the need for long-life batteries, or can be used to supplement or charge the batteries. In either case, the sensor avoids the need for large batteries and is therefore small enough for installation in the valve stem or valve itself.

OPTIONAL AND PREFERRED FEATURES

Preferable and optional features of the various broad aspects of the invention are set out below. The skilled worker in the field will understand that while some of the features described below are optional for some of the above broad aspects of the invention, they are essential to other broad aspects.

Preferably the sensor is powered by radio waves transmitted from a remote source. Preferably the sensor is a capacitative pressure sensor with a conductive layer within the chamber spaced from the flexible membrane such that they form opposing electrodes of a capacitor. In a further preferred form the conductive layer is less than 50 microns from the membrane in its undeflected state.

Preferably, the membrane is circular with a diameter less than 500 microns. In a further preferred form the membrane is less than 300 microns and in specific embodiments the diameter is 100 microns.

In some preferred embodiments the membrane is approximately 0.5 μm thick. In further embodiments, the membrane is a 100 micron diameter circular film. Preferably the metal ceramic is a metal nitride. In specific embodiments, the membrane is titanium nitride, tantalum nitride, and vanadium nitride. The membrane may also be form from mixed metal nitrides. The mixed metal nitrides may be titanium silicon nitride, tantalum silicon nitride, vanadium silicon nitride, titanium aluminium silicon nitride, tantalum aluminium silicon nitride and so on.

Preferably, the flexible membrane is a laminate having at least two layers wherein at least one of the layers is at least partially formed from conductive material. The layers within the laminate may be formed from the deposition of different metal ceramics. Preferably, the metal ceramics are metal nitrides or mixed metal nitrides, such as titanium nitride, titanium aluminium nitride, tantalum silicon nitride, titanium aluminium silicon nitride, tantalum aluminium silicon nitride and so on. Layers of the laminate may also be metal such as titanium or vanadium.

In a particularly preferred form, the sensor further comprises a second chamber with a second membrane, the second chamber being sealed from the fluid pressure and the second membrane deflecting from a predetermined pressure difference in the second chamber; wherein,

-   -   the associated circuitry converts the deflection of the second         membrane into an adjustment of the output signal to compensate         for the temperature of the sensor.

In some embodiments, the sensor is formed on and through a silicon wafer using lithographically masked etching and deposition techniques. In a further preferred form, the sensor is a capacitative sensor, wherein a conductive layer is deposited in each of the first and second chambers and the first and second flexible membranes are conductive, such that, the conductive layer in the first chamber and the first flexible membrane form capacitor electrodes wherein the deflection of the first flexible membrane changes the capacitance which the associated circuitry converts to the output signal.

Preferably, there is provided a CMOS layer disposed between the metallic layer and the substrate. In some embodiments, the sensor is additionally provided a passivation layer at least partially deposited over the metallic layer.

Optionally, the pressure sensor is adapted to sense the air pressure within a pneumatic tire.

Conveniently, the wafer is a first wafer substrate with a front side and an opposing back side, a recess etched into the front side and at least one hole etched from the back side to the recess; and the sensor further comprises:

-   -   a second wafer on the back side of the first wafer to seal the         at least one hole; wherein,     -   the second is wafer bonded to the wafer substrate.

Optionally, the wafer bonding is direct wafer bonding wherein the contacting surfaces of the first and second wafers are ultra clean, and activated by making them hydrophilic or hydrophobic prior to bonding, and then brought into contact at high temperature, preferably around 1000° C. Anodic bonding offers another option wherein the contacting surfaces of the first and second wafers have a large voltage applied across them. The wafers may be in a vacuum, air or an inert gas when the bond is formed. Intermediate layer bonding is a third option wherein a layer of low melting point material is applied to one or both of the contacting surfaces of the first and second wafers so that heat and pressure forms the wafer bond. Preferably the low melting point material is silicon nitride or titanium nitride. This option avoids the high surface cleanliness required by direct silicon bonding and the high voltages required by anodic bonding.

In some embodiments, the membrane is non-planar and preferably corrugated. In a further preferred form, the flexible corrugated membrane has a corrugation factor of 8. In a particularly preferred form, the sensor has a linear response up to about 1 MPa. In specific embodiments, the membrane corrugations have a period of between 5 microns and 15 microns, preferably about 10 microns. In some preferred embodiments, the membrane is substantially circular and the corrugations are annular. In a particularly preferred form, the corrugations have a substantially square-shaped cross-sectional profile.

Preferably the sensor further comprises an apertured guard over the membrane formed using lithographically masked etching and deposition techniques. In these embodiments, the guard is laminate having at least two layers. In a particularly preferred form the layers are different materials such as silicon nitride and silicon dioxide.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present invention is now described by way of example only, with reference to the accompanying drawings in which:

FIG. 1 (Prior art—http://www.pacific-ind.com/eng/products2/carsystem.html) illustrates a typical TPMS used in a four wheel vehicle;

FIG. 2 is a schematic partial section of a sensor according to one embodiment of the present invention;

FIGS. 3, 5, 8, 10, 13, 15, 18 and 20 are schematic plan views of masks suitable for use in particular stages of the fabrication process;

FIGS. 4, 6, 7, 9, 11, 12, 14, 16, 17, 19, 21, 22 and 23 illustrate sections of an embodiment at successive stages of fabrication;

FIG. 24 is a schematic partial section of a reference sensor for providing temperature compensation for a sensor according to present invention;

FIGS. 25 a to 25 d are a schematic representation of the installation of the sensor in the valve stem of a vehicle tire;

FIG. 26 is a diagrammatic representation of the sensor circuit;

FIGS. 27 a to 27 h are partial perspectives (some partially sectioned) of a sensor according to an embodiment of the present invention;

FIGS. 28 a (perspective view) and 28 b (side view) illustrate a schematic of a membrane with corrugations according to a possible embodiment of the present invention;

FIG. 29 illustrates a comparison of linear and non-linear theory with linear and non-linear finite element modelling for a flat circular titanium nitride (TiN) membrane, with R=50 μm and t=0.5 μm;

FIG. 30 illustrates the effect of membrane corrugations on pressure-deflection sensitivity;

FIG. 31 illustrates an expanded view of Von Mises Stress distribution near an edge of a membrane, with p=1 MPa, q=4 and l=10 μm; and

FIG. 32 illustrates the effect of temperature on the pressure-deflection response of a membrane with q=8 and l=10 μm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following embodiments are described in order to provide a more precise understanding of the subject matter of the present invention. While the embodiments focus on a capacitative type sensor, ordinary workers in this field will readily understand that the invention is equally applicable to other forms of pressure sensor such as:

-   -   (i) Piezo-resistive, where the membrane is formed from a         non-conductive material and the piezo material is in contact         with the membrane. Deflections of the membrane give rise to         piezo-induced changes in resistivity (and hence current, if a         voltage is applied) that can be monitored electronically.     -   (ii) Resonant pressure sensors, where the frequency of         oscillation of the membrane depends on the pressure difference.         The initial resonance could be activated by using a time-varying         electrostatic force between the two electrodes.     -   (iii) Force compensation pressure sensors, where an         electrostatic force is applied to maintain the membrane at the         initial position. Once again the electrostatic force between the         two electrodes can be used to position the membrane.         Each of these sensor types has particular advantages and         limitations. Piezo-resistive sensors are reasonably well known         and understood but require the use of exotic materials. The         sensors of (ii) and (iii) are less popular but do not require         exotic materials. Capacitative sensors are typically robust and         highly versatile and therefore the preferred embodiments will be         based on this type of sensor.         Functional Overview         A brief overview of the basic operation of the sensor will be         described with reference to FIG. 2. FIG. 2 shows a schematic         partial section of a capacitative sensor fabricated using the         masked lithographic etching and deposition techniques typically         used in the production of semiconductor chips. The fabrication         steps are described in detail below.         The sensor 30 is formed on a silicon substrate 32, provided with         sealed channels or holes 33, on which is deposited a CMOS layer         34. A conductive layer 36 is deposited on the CMOS layer 34         followed by a passivation layer 37 as illustrated. The         passivation layer 37 may be an insulating or semi-conducting         material. A conductive membrane 50 is spaced from conductive         layer 36 to form a reference chamber 58. Roof or cap 54 covers         the membrane 54. The roof or cap 54 is provided with holes 40,         or channels or the like, so that the membrane 50 is exposed to         tire pressure.         The membrane 50 deflects due to differential stresses. The         amount of deflection for a given pressure difference, depends on         the membrane diameter and thickness, the nature of the support         (for example, strongly fixed, weakly pinned), and the membrane         material properties (for example elastic modulus, Poisson ratio,         density).         Both the membrane 50 and conductive layer 36 are electrodes,         which develop a capacitance, C, between them which depends upon         the electrical permittivity of the ambient material, e, the         electrode spacing, d, and the electrode area, A. For the case         where both electrodes are circular disks, C=e A/d. The sensor is         then calibrated for measured capacitance versus applied         pressure.         Fabrication Overview         FIGS. 4, 6, 7, 9, 11, 12, 14, 16, 17, 19, 21, 22 and 23 show the         main lithographic etching and deposition steps involved in the         fabrication of a pressure sensor according to the invention. The         masks associated with the successive steps are shown in FIGS. 3,         5, 8, 10, 13, 15, 18 and 20. When etching photoresistive         material the solid black areas of the masks are the regions that         are removed by the subsequent etch. However, when etching metal         and other non-photoresistive layers, the blank or unmasked areas         of the mask denote the regions that are removed. Skilled workers         in this field will understand which masks are applied to         photoresist and which are applied to non-photoresist.         FIG. 4 is section A–A′ through the wafer 32 of a partially         fabricated sensor. The silicon wafer 32 has previously been         ground to the required thickness and CMOS circuitry 34 is         deposited on its top surface. The final CMOS layer provides the         bottom electrode 36 for the sensor. Guard rings 42 are formed in         the metallization layers of the CMOS circuitry 34. The guard         rings 42 prevent air or other fluid from diffusing from the         subsequently etched sealed passages 33 (see FIG. 2) through the         wafer 32 to the circuitry where it can cause corrosion. The mask         70 for this first metal layer 36 is shown in FIG. 3, with the         blank regions being etched away.         As shown in FIG. 6, a passivation layer 37 and sacrificial layer         38 are deposited next. This is followed by masking and etching         through to the silicon substrate 32. This etch is known as the         dielectric etch and the associated mask 72 is shown in FIG. 5.         The mask represents the regions 46 that are etched. Following         the dielectric etch, the sacrificial layer 38 is etched away         with a different etchant which also etches the holes 33 deeper         into the wafer substrate 32 (see FIG. 7).         Referring to FIGS. 8 and 9, the passivation layer 37 is then         etched in a region 44 above the upper contact to provide an         electrical pathway for the second electrode (subsequently         deposited).         As shown in FIG. 11, sacrificial material 48 is then deposited         to fill the openings 33 into the silicon 32 made by the         dielectric etch. This deposition continues until the top of the         sacrificial layer 48 is level and at a height which provides the         requisite gap height between the top and bottom electrodes. The         first sacrificial layer 48 is then patterned and etched. The         associated mask 76 is shown in FIG. 10.         FIG. 12 shows the deposition of the upper electrode layer 50,         also called the second metal layer. This layer is etched with         the mask 78 shown in FIG. 13, with the blank regions being         removed by the etch (see FIG. 14). The upper electrode layer 50         becomes the flexible membrane in the finished sensor.         A second sacrificial layer 52 is then deposited (see FIG. 16),         and subsequently etched. The relevant mask 80 is shown in FIG.         15.         FIG. 17 shows the deposition of the roof layer 54. It is then         etched using mask 82 shown in FIG. 18, with blank areas removed         (see FIG. 19).         The wafer is subsequently turned over for ‘back etching’ from         the reverse side of the wafer 32. FIG. 21 shows a deep back etch         56 extending through to meet the openings 33 in accordance with         the mask 84 shown in FIG. 20. The openings 33 are filled with         sacrificial material 48 which is exposed by the deep back etch         56. The sacrificial material is removed by plasma cleaning (see         FIG. 22) through the deep etch 56.         As shown in FIG. 23, the wafer is again turned over and the         sacrificial material 52 is removed through the hole in the roof         layer 54. To complete the device, it needs to be packaged, with         the bottom face of the wafer being sealed. Skilled workers will         appreciate that there are various methods of achieving this.         However, in the preferred embodiment, the bottom face of the         wafer is sealed using wafer bonding, which is discussed in         detail below.         Temperature Compensation         Differential thermal expansion of the components within the         pressure sensor will affect the membrane deflection and         therefore the sensor output. If the sensor is used in an         environment with a large temperature variation, the sensor         accuracy can become unacceptable. To address this, the pressure         sensor can be coupled with a temperature sensor, with the         pressure sensor then calibrated as a function of temperature.         To accommodate this, an embodiment of the present invention can         conveniently incorporate a temperature sensor to account for         temperature effects on the pressure sensor. A schematic section         of the temperature is shown in FIG. 24. Reference chamber 58,         can be etched into the same wafer substrate, but is not exposed         to tire pressure like the adjacent pressure sensor (not shown).         In these embodiments, the coupled sensors form an active and a         reference sensor, the latter responding only to thermal changes.         Skilled workers in this field will appreciate that the reference         sensor can also serve as a temperature sensor with the addition         of circuitry calibrating the capacitance to the temperature.         Referring to FIG. 24, the reference sensor is made in the same         way as the active sensor, except that the holes 40 are made in         the membrane 50 instead of the roof layer 54. The sacrificial         material 52 between the membrane 50 and the roof layer 54 is         removed with a back etch through the holes 40 in the membrane         50.         An alternative to this is to keep the membrane 50 intact and         etch away the second sacrificial layer material 52 from above         the active part of this layer, before deposition of the roof         layer 54. This causes the membrane 50 to be bonded to the roof         54, and this configuration is much stiffer. Therefore, the exact         dimensions of the reference sensor would need to be adjusted to         provide a similar capacitance change in active and reference         sensors due to thermally induced stress changes in the membrane         50.         Temperature Compensating Sensor Design         FIGS. 27 a to 27 h show perspectives of temperature compensating         sensor at various stages of fabrication. As best shown in FIGS.         27( a) to 27(d), the reference sensor 51 is etched into the same         wafer substrate 32 as the active sensor 53. This embodiment has         further increased the structural strength by adding the top         cover 60 over the roof layers 54 of the active and reference         membranes. The cover 60 defines separate chambers 62 and 64 over         the reference and active roofs 54 respectively. In this         embodiment, the roof layers 54 of each sensor stop smaller         particles from contacting the membranes 50. The top cover 60         provides much greater structural rigidity while protecting the         membrane and roof guard layers 54 from damaging contact during         installation. However, even with the top cover 60, the sensor         has overall dimensions that are small enough for installation in         the tire valve or valve stem.         As best shown in FIG. 27 c, chamber 62 is sealed from the tire         pressure, whereas chamber 64 is exposed to the tire pressure via         vent 66 and channel 68. While it has not been shown in the         figures, it will be appreciated that the vent 66 extends through         to the back surface of the wafer substrate 32, where it is not         sealed, but open to the tire pressure. If the sensor is wafer         bonded to a sealing wafer (as discussed below), the sealing         wafer has corresponding holes for establishing a fluid         connection with the tire pressure.         Semiconductor Fabrication Techniques         Using the lithographically masked etching and deposition         procedures of semiconductor fabrication, it is possible to         manufacture a robust, low cost tire pressure sensor from         Micro-Electro-Mechanical (MEM) based devices for use in a TPMS.         The membrane can be formed from a material that is capable of         withstanding a wide range of environmental conditions. An         advantage of such a tire pressure sensor is the relatively low         cost of manufacture. The membrane can be formed in many possible         geometries, for example, as a generally flat or planar shape         such as a disc, or having a featured surface.         Sensor Circuitry         FIG. 26 is a diagrammatic representation of a capacitance         sensing circuit where:     -   C_(s) is the capacitance of the sensor capacitor;     -   C_(r) is the capacitance of the reference capacitor (preferably         made in the same way as sensor capacitor but non-sensing); and     -   C_(p) is a parasitic capacitance to ground.         V1 and V2 are constant voltages reference voltages provided to         switches 80. These may be chosen to be two of the circuit power         supplies or internal regulated supplies. These voltages are then         switched onto one of each of the capacitor plates charging or         discharging them. The voltage V_(in) is fed to charge amplifier         84, a high gain device, which amplifies the voltage to V_(out).         V_(in) provides a measure of charge imbalance in the circuit         when it is operated as follows:         Step 1.         Connect C_(r) to V2, C_(s) to V1; reset the charge amplifier 84         which forces V_(in) to a fixed voltage V_(r) and set the charge         injector to a know state with charge Q₁₁.         The total stored charge, Q₁, is:         Q ₁ =C _(r)(V2−V _(r))+C _(s)(V1−V _(r))+Q ₁₁         Step 2.         Connect C_(r) to V1, C_(s) to V2, and remove the reset from the         charge amplifier 84. The output from the charge amplifier 84 is         monitored by the control 86 and feedback applied to the charge         injector such that the total charge is balanced forcing         V_(in)=V_(r) by injecting a charge of Q₁₁−Q₁₂.         The total stored charge, Q₂, is given by:         Q ₂ =Cr(V1−V _(r))+C _(s)(V2−V _(r))+Q ₁₂         Feedback forces Q₁=Q₂, so that the digital output from the         control 86 is:         Q ₁₁ −Q ₁₂=(V1−V2).(C _(r) −C _(s))         The control logic 86 may operate an iterative procedure to         determine the required output to obtain this charge difference,         at the end of which it will produce the required digital output.         The voltage on C_(p) is the same at the end of Step 1 and Step         2, and so ideally, does not contribute to the digital output.         Optionally, these steps can be repeated and an averaging applied         to the digital output to reduce random noise. Furthermore,         additional steps may be added to the above idealized case, in         order to improve accuracy of the circuit.         Sensor Installation and Power Supply         FIGS. 25 a to 25 d schematically show the installation of the         sensor within the valve stem of a car tire. The pressure sensor         can be mounted in other locations including the valve head, the         tire wall, the wheel hub and so on. However, the relatively         solid structure of the valve stem makes it the most convenient         component for automated installation of the sensor. Furthermore         as the stem closer to the centre of the wheel than the rest of         the tire, the acceleration forces on the sensor generated by         wheel rotation are less.         As best shown in FIGS. 25 b and 25 c, the sensor 96 is mounted         in the valve stem 92. A small recess is created in the valve         stem wall 94. A thin layer of hard-setting adhesive 98 is         applied as a coating on the recess walls. The sensor chip 96 is         then adhered into the recess with any excess adhesive removed         before the adhesive 98 is cured.         Power can be supplied to the sensor chip 96 in a number of         fashions, including, but not limited to, a long-life battery         (not shown) located in the valve stem wall 94, a long-life         battery located in the valve head 92, or radio frequency energy         beamed to an electromagnetic transducer from an external         station. The embodiment shown in FIGS. 25 a to 25 d is the         latter. The pressure and temperature are sampled once per         second, or at any other rate as required by legal or commercial         obligations, and the results are displayed on the dashboard tire         sensor display 106, marked as level monitors 104. If the tire         pressure is outside the levels specified by the tire         manufacturer for proper tire functioning, or indeed any other         limits which arise from legal or commercial obligations, the         specific tire 90, or tires, will have an error shown in colour         (eg. red) on the car chassis symbol 102.         If the power is supplied to the sensors 96 by long-life         batteries, the sensor display 106 would include a low battery         indicator. This can be conveniently by illuminating the problem         signal 102 in a different colour (eg. purple).         Low Mass/Conductive Ceramic Membrane         In a particular embodiment, the sensor has a membrane that is at         least partially a conductive-ceramic compound, for example,         titanium nitride, TiN. The use of MEMS-based sensors reduces the         effects of acceleration due to the greatly decreased mass. As an         illustrative but non-limiting example, a TiN membrane, with         density of 5450 kgm⁻³, radius of 50 μm and thickness of 0.5 μm,         should experience a force of 0.2 μN due to an acceleration of         1000 g; compared with a force of 1.6 mN for a pressure of 207         kPa (approximately 30 psi), which is typical for standard tire         inflation pressure. The low mass of the membrane ensures that         the affect of acceleration is negligible compared to that of the         pressure.         TiN has been found to have a surprisingly high yield strength         compared with other known materials used for capacitive sensor         membranes. This makes it suitable for use in a wider range of         stressful, harmful or dangerous environments. This also means         that under standard conditions, membranes made from TiN should         have greater lifetimes compared with standard capacitive         pressure sensors.         The sensor membrane may be composed of other conductive-ceramic         compounds. In particular, metal-ceramics, for example titanium         aluminium nitride (TiAIN), titanium aluminium silicon nitride         (TiAlSiN), tantalum nitride (TaN), tantalum aluminium nitride         (TaAIN) or the like, have suitable properties. These metal         ceramics are hard wearing and form a tough, thin surface oxide.         The protective oxide gives the sensors good robustness and         longevity. Metal ceramics are well suited to deposition by         semiconductor fabrication techniques. This allows the sensor to         have a thin membrane (0.5 μm to 5 μm) with diameters ranging         from about 20 μm to 2000 μm. As discussed below, thin membranes         have less internal stresses from rapid cooling, and less mass         for limiting acceleration effects.         Squeeze Film Damping         According to another possible embodiment, an electrode in the         sensor can be provided with holes to prevent squeeze film         damping. Squeeze film damping occurs when the membrane deflects         very close to the static electrode. The pressure of the fluid         between the membrane and the static electrode rises and         restricts, or damps, the membrane motion. This can severely         restrict the dynamic response of the membrane.         Channels or apertures can let the fluid between the fixed         electrode and the dynamic electrode (membrane) flow away from         the narrowing space between them. The fluid must still remained         sealed within the sensor, however, letting it escape from         between the opposing faces of the membrane and the fixed         electrode avoids squeeze film damping effects.         Internal Stresses         Residual stress in the membrane can affect its deflection and         therefore the accuracy of the sensor. The fabrication process is         a typical cause of residual stress. Rapid cooling material from         an elevated temperature, can generate thermal stresses in         proportion to the material co-efficient of thermal expansion.         These stresses can deform the membrane and change its deflection         characteristics under fluid pressure. This in turn can affect         the accuracy of the pressure reading.         As discussed above, masked lithographic deposition of the         conductive-ceramic membrane allows it to be very thin—typically         of the order of 0.5 μm thick. The temperature profile across the         thickness of a thin membrane is much flatter than that of a         relatively thick membrane. The membrane cooling is far more         uniform and the internal stresses are largely eliminated.         Laminated Membrane         Masked lithographic deposition also allows the sensor to have a         relatively thick membrane for harsh operating conditions, while         still avoiding the problems of residual thermal stresses. By         forming the membrane as a laminate, the separately deposited         layers are individually thin enough to avoid residual stress but         the final laminate is sufficiently strong.         Instead of depositing layers of the same conductive material,         the individual layers can be selected so their collective         properties provide good resistance to harsh environments. For         example alternate layers of TiN or TiAlN or various other         combinations of metals and ceramics, for example, Ti and TiN.         Laminated Roof Layer         Likewise the roof layer 54 may also composed of several         different material layers, for example, silicon nitride, Si₃N₄,         and silicon dioxide, SiO₂. Again, this avoids residual thermal         stresses.         Simulation Results and Analysis         To permit an examination of the performance of a TiN membrane         capacitive sensor, a series of numerical simulations have been         performed using a commercial finite-element modelling package         called ANSYS 5.7 (http://www.ansys.com). Axisymmetric models         were used to reduce computational time. This required a symmetry         boundary condition at the centre of the membrane, whilst the         edge of the membrane was held fixed. A square mesh with one         thousand nodes distributed equally across the radius was         employed, and meshes with half the number of cells exhibited         less than 5% difference in the maximum stress and deflection.         As discussed above, a MEMS fabrication procedure can be used to         deposit a 0.5 μm thick layer of TiN to form a membrane. A         membrane deflection of 5 μm provides sufficient variation in         capacitance. For a standard passenger vehicle, the pressure         applied to the membrane is typically be in the range 0–45 psi,         allowing for 50% tire over-inflation. According to linear theory         (see Young, W. C. and Budynas, R. G., Roark's Formulas for         Stress and Strain, 7^(th) Edition, p 488, 2002), the membrane         radius, R, for a specific deflection, Δ, and applied pressure,         P, is given by:

$\begin{matrix} {R = \left\{ \frac{16\Delta\;{Et}^{3}}{3{P\left( {1 - v^{2}} \right)}} \right\}^{\frac{1}{4}}} & (1) \end{matrix}$ where E is the modulus (approximately 500 GPa for TiN), t is the membrane thickness and v is Poisson's ratio (0.25 for TiN). For these values it is found, from equation (1), that R≈50 μm. The variation of membrane deflection with applied pressure is shown in FIG. 3 for a membrane radius of 50 μm. The comparison between linear finite element model (FEM) and linear theory is quite good. At very high pressure, the deflection-pressure response becomes non-linear, and it is important to include these non-linear effects when designing a working tire pressure sensor. Non-planar Membrane It is possible to extend the linear range of the device by corrugating the membrane. FIGS. 28 a and 28 b illustrate a possible embodiment of a non-planar membrane 50 for use in a TPMS sensor. The membrane 50 is generally circular in extent and is provided with corrugations formed as annular ridges 22 on a base region 24. The number and spacing of annular ridges, and the individual shape of the annular ridges 22, can vary. Also, differently shaped annular ridges 22 could be provided on a single base region 24. In the particular embodiment illustrated, the corrugations are formed to have a square-shaped cross-sectional profile. Geometric parameters s, l, H and t are also illustrated and are referenced in the following equations. For a circular membrane, this amounts to superimposing a series of raised annuli on the membrane profile as illustrated in FIG. 28 b. A theoretical model for the non-linear response for a corrugated membrane is:

$\begin{matrix} {\frac{{PR}^{4}}{{Et}^{4}} = {{A_{p}\left( \frac{\Delta}{t} \right)} + {B_{p}\left( \frac{\Delta}{t} \right)}^{3}}} & (2) \end{matrix}$ where

$\begin{matrix} {A_{p} = \frac{2\left( {q + 2} \right)\left( {q + 1} \right)}{3\left\{ {1 - \left( {v/q} \right)^{2}} \right\}}} & (3) \\ {{B_{p} = {\frac{32}{q^{2} - 9}\left( {\frac{1}{6} - \frac{3 - v}{\left( {q - v} \right)\left( {q + 3} \right)}} \right)\mspace{14mu}{and}}}\mspace{14mu}} & (4) \\ {q = {\frac{s}{l}\left\{ {1 + {1.5\left( \frac{H}{t} \right)^{2}}} \right\}}} & (5) \end{matrix}$ The variable q is referred to as the corrugation quality factor, s is the corrugation arc length, l is the corrugation period and h is the corrugation height (refer to FIG. 28 b). For right-angled corrugations, s=l+2h. For a flat membrane, q=1. To include non-linear effects in the finite element calculation, the load is applied over a number of sub-steps and an equilibrium solution is sought for each sub-step. The results for the non-linear simulation and theory are also shown in FIG. 29. The response becomes non-linear at approximately 30 kPa, which is well below the maximum expected tire pressure. The non-linear finite element simulations match the linear and non-linear theories below and above the critical point, respectively. To assess the effect of corrugations on sensor designs, finite element models were constructed for two different corrugation periods, l=10 and 20 μm, and two different quality factors, q=4 and 8. This results in a corrugation height of approximately 0.65 and 1.0 μm for q=4 and 8, respectively. The results in FIG. 30 indicate that it is necessary to include non-linear effects for the pressure range considered here; non-linearity should also be present due to the large number of rigid corners in the model. The results of the finite element simulations are compared with the theoretical model in FIG. 30. This shows that a corrugation factor of 8 will extend the linearity of the sensor up to an applied pressure difference of approximately 1 MPa. It also shows that the corrugation period does not have a strong effect for the configurations examined herein. The maximum Von Mises stress behaves in a similar manner to the membrane deflection. The stress is concentrated near the junction at the lower side of the outermost corrugation with progressively less stress on the inner corrugations (see FIG. 31 for a typical stress distribution). The coefficient of thermal expansion for TiN is 9.4×10⁻⁶ K⁻¹, which means that, for the range of likely operating temperatures, thermally induced stress might alter the sensor deflection. The effect of temperature on the sensor response is examined in FIG. 32 for a sensor with q=8 and l=10 mm. The range of temperatures T examined, from −20 to +40° C., can be considered as representative of the heat-up cycle which occurs when a vehicle initiates a journey in a cold environment. It is seen that below 100 kPa pressure difference, there is a strong effect of temperature. At higher applied pressures, typical of operation, the thermal stresses are swamped by pressure-induced stresses and the temperature has little effect on the sensitivity. Thus, a corrugated membrane sensor has a linear response in the region of interest. The small size of the sensor means that it is suitable for installation in wheel hub/rim valve stem and valve cap systems. Wafer Bonding As discussed above in relation to FIGS. 3 to 23, the deep etch hole 56 needs to be sealed to maintain the fluid beneath the membrane 50 at a reference pressure. Typically, the reverse side of the wafer 32 is bonded to a sealing wafer. Unfortunately, simply using a polymeric adhesive to bond a main wafer to the sealing wafer is not sufficient. The reference cavity (i.e. fluid beneath the membrane 50) seeks to maintain a constant pressure to ensure a minimum of calibration drift. The polymeric adhesive is permeable to air, which may result in leakage of air into the reference cavity because of the tire pressure. The flow rate across a permeable material is given by:

$\begin{matrix} {Q = \frac{P_{12}A\;\Delta\; P}{L}} & (6) \end{matrix}$ where P₁₂ is the permeability of the material, A is the surface area of the material exposed to the pressure difference, ΔP is the pressure difference and L is the flow path length. The permeability of most polymers is of order 10⁻²¹ m³ (STP) ms⁻¹m⁻²Pa⁻¹, so for a pressure difference=300 kPa, cavity radius=50 μm, seal height=10 μm and seal (flow) length=10 μm, the flow rate is approximately 9.4×10⁻²⁰ m³ s⁻¹. If the cavity height is 100 μm, then the total cavity volume is 7.9×10⁻¹³ m³, and approximately 100 days would be required for the reference cavity pressure to equilibrate with the tire pressure. Whilst this may be suitable for testing purposes, it would make such a device unsuited to practical use. Another way to seal the reverse side of the wafer is to over-mold the sensor with plastic to increase the path length for leakage. The permeability of polymers is about ten times less than that of adhesives, and so a ten-year seal would require a seal length of approximately 0.5 mm. This is too long for a MEMS device that is less than 50 μm long. To maintain the high yield and versatility of the pressure sensor as a MEMS device, a different sealing solution is required. Wafer bonding offers the possibility of a hermetic seal at the cost of a slightly different fabrication procedure. The most prevalent forms of wafer bonding are: direct wafer bonding, anodic (electrostatic) bonding and intermediate layer bonding (see Table 3).

TABLE 3 Wafter bonding techniques. BCB = Benozcyclobutene, PDMS = polydimethylsiloxane, PSG = phosphosilicate glass. * = unsuitable due to porosity. T_(bond) Surface Type of bond Description (° C.) Finish Direct Wafer Reactive surfaces brought  800–1100 <0.01 μm (Si Fusion) into contact under high temperature Anodic High voltage (0.5 to 1.5 kV)  200–1000 <1 μm (Electrostatic, applied across both wafers Mallory Process) Intermediate Low melting temperature  1 μm Layer material applied to one or Au—Si both wafer surfaces, bonds Glass Frit form when temperature and 363 (e.g. pressure are applied 400–600 Corning #75xx) Au—Au BCB* PDMS (plasma <400 treated) Tin, Si₃N₄ 250 LPCVD PSG 25 APCVD BO Boron-  100–300? Doped Si Organics* 1100 450 450 <200 In the first method, two ultra clean surfaces are brought into contact after they have been activated (e.g. the surfaces are made hydrophilic or hydrophobic); the bond forms at elevated temperatures (near 1000° C.). In the second, the two wafers are brought into contact, in either vacuum, air or an inert atmosphere, and a large voltage is applied across the wafers. The bond forms because of migration of ions from one wafer surface to the other. This method has relaxed requirements in terms of surface finish, which makes it more suitable for bonding two wafers that have undergone a series of fabrication steps; however, the high voltage may damage CMOS layers. The third option employs a layer of low melting point material that is deposited on one or both wafers on the contact face. The wafers are brought into contact at moderate temperatures and the bond forms at the interface once pressure is applied. There are many different materials that may be used to form the intermediate layer (e.g. Si₃N₄ TiN). The intermediate layer method overcomes the disadvantage of direct wafer (high cleanliness) and anodic (high voltage) bonding. The invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. Although the preferred embodiment has been described in detail, it should be understood that various changes, substitutions, and alterations can be made by one of ordinary skill in the art without departing from the scope of the present invention. 

1. A pressure sensor comprising: a sensor membrane exposed to fluid at a known pressure on one side, and exposed to fluid at a pressure to be sensed on its opposite side, the sensor membrane being at least partially formed from a conductive material; a reference membrane exposed to a first reference pressure on one side, and exposed to a second reference pressure on its opposite side, the reference membrane being at least partially formed from a conductive material; associated circuitry connected to both the sensor membrane and the reference membrane for converting deflection of the sensor membrane into an output signal indicative of a pressure difference between the known pressure and the fluid pressure to be sensed; wherein, the associated circuitry converts deflection of the reference to a temperature compensation value used to adjust the output signal to provide a temperature compensated output indicative of the pressure to be sensed.
 2. A pressure sensor according to claim 1 wherein the first reference pressure and the second reference pressure are equal.
 3. A pressure sensor according to claim 2 wherein the known pressure is equal to the first and second reference pressures.
 4. A pressure sensor according to claim 1 wherein the sensor membrane partially defines a first chamber containing the fluid at the known pressure, and the reference membrane divides a second chamber containing the first and second reference pressures.
 5. A pressure sensor according to claim 1 wherein the sensor and reference membranes, and the associated circuitry are formed on and through a wafer substrate using lithographically masked etching and deposition techniques.
 6. A pressure sensor according to claim 1 wherein the sensor membrane is less than 0.1 grams.
 7. A pressure sensor according to claim 1 adapted to sense the air pressure within a pneumatic tire.
 8. A pressure sensor according to claim 4 wherein the sensor is a capacitative sensor having: a conductive layer deposited in the first chamber to form a first capacitor electrode; and, the sensor membrane is a second capacitor electrode; such that, deflection of the sensor membrane changes the capacitance which the associated circuitry converts to the output signal.
 9. A pressure sensor according to claim 1 wherein the sensor and reference membranes are at least partially made from a metal-nitride compound.
 10. A pressure sensor according to claim 9 wherein the metal-nitride compound is titanium nitride, titanium aluminium nitride, titanium aluminium silicon nitride, tantalum nitride or tantalum aluminium nitride.
 11. A pressure sensor according to claim 10 further comprising a CMOS layer disposed between the conductive layer and the substrate.
 12. A pressure sensor according to claim 11 further comprising a passivation layer at least partially deposited over the conductive layer.
 13. A pressure sensor according to claim 1 wherein the sensor and reference membranes are each approximately 0.5 μm thick.
 14. A pressure sensor according to claim 1 wherein the conductive layer is less than 50 microns from the sensor membrane in its undeflected state.
 15. A pressure sensor according to claim 1 wherein the membrane is circular with a diameter less than 500 microns.
 16. A pressure sensor according to claim 1 wherein the sensor is powered by radio waves transmitted from a remote source.
 17. A pressure sensor according to claim 1 wherein the sensor and reference membranes are each a laminate having at least two layers wherein at least one of the layers is at least partially formed from conductive material.
 18. A pressure sensor according to claim 17 wherein the metal ceramics are metal nitrides or mixed metal nitrides.
 19. A pressure sensor according to claim 18 wherein, the metal ceramics are titanium nitride, titanium aluminium nitride, tantalum silicon nitride, titanium aluminium silicon nitride, tantalum aluminium silicon nitride.
 20. A pressure sensor according to claim 17 wherein one or more of the layers of the laminate is metal. 